Network security lab certification 350 018

Exam : 350-018(LAB)

Title : Network Security

Ver : 12.19.06

350-018

• Network Security

Lab Abstract

Currently your routers have no security features configured on them beyond enable secret passwords and login
passwords on the vty lines for Telnet access. All of the vty lines share the same password. You decide to
implement some security features. Here is what you want to do:

1. Configure each of the routers with passwords for Console access.
2. "Reserve" one vty line on each router for your own access by setting a different password on it.
3. Change the enable secret password on all the routers.
4. Configure access lists on each router to allow Telnet connections only from your workstation (IP address
172.18.56.14)
5. Configure access lists on each router to deny all ping requests sent to the routers from workstation (IP address
172.18.56.16)
6. Log any traffic that is denied by the access lists that you implement.
7. Make sure that no other network traffic is impacted by the implementation of these access lists.

Lab Scenario

Network Security Lab Scenario
Introduction
You are the network administrator for The Meely Meal company. Owned by Milton Meely, the
company is a leading distributor of wheat germ and other grains and cereals. The company has three
locations:

1. Corporate Headquarters in Albuquerque, New Mexico.

2. A packaging and distribution plant in Battle Creek, Michigan.

3. A small purchasing office in Lincoln, Nebraska.

A diagram of the network is included below.

Milton has hired his son, Matt, as an intern for the summer. Matt tells you that he is thinking of getting
his CCNA. He says that he plans to prepare by reading "the" book. You tell him that it might be a
good idea to get some hands on experience before taking the test. Milton thinks is a great idea.
Suddenly Matt is your new "assistant" and wants to have access to the company routers so he can
play with them.

Needless to say, you are concerned, and you want to limit the access that he has. You are willing to
teach him IOS commands as long as you are standing with him while he connects to the local router

CertGuaranteed. Study Hard and Pass Your Exam

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through the console port, but you do not want him accessing the routers remotely while you are not
around.

Currently the routers have no security features configured on them beyond enable secret passwords
and login passwords on the vty lines for Telnet access. All of the vty lines share the same password.
Here is what you want to do:

Objectives
1. Configure each of the routers with passwords for Console access.

2. "Reserve" one vty line on each router for your own access by setting a different password on it.

3. Change the enable secret password on all the routers.

4. Configure access lists on each router to allow Telnet connections only from your workstation (IP
address 172.18.56.14).

5. Configure access lists on each router to deny all ping requests sent to the routers from Matt's
workstation (IP address 172.18.56.16).

6. Log any traffic that is denied by the access lists that you implement.

7. Make sure that no other network traffic is impacted by the implementation of these access lists.

Network Diagram


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Solution
1. Login to each router and enter Privileged Exec mode. Enter Global configuration mode with the
configure terminal command. Use the line con 0 command to configure the console line. Use the
login and password commands to configure the console for login with a password. Here is an
example using the Battle Creek router:

Battle>enable
Password:*******
Battle#conf term
Battle(config)#line con 0
Battle(config-line)#login
Battle(config-line)#password oatmeal
Battle(config-line)#^Z

2. While logged into the router, enter Privileged Exec mode. Then enter Global Configuration mode.
Use the line vty command to configure the virtual terminal lines. First configure lines 0 through 3
using the line vty 0 3 command. Assign a password to these four lines. Then configure the last line
with a different password using the line vty 4 command. Here is an example on the Battle Creek
router:

Battle>enable
Password:*******
Battle#conf term
Battle(config)#line vty 0 3
Battle(config-line)#password oatbran

Battle#conf term
Battle(config)#line vty 4
Battle(config-line)#password shellfish

3. Connect to the router, and enter Global Configuration mode. Use the enable secret command to
change the enable secret password. Here is an example:

Battle>enable
Password:*******
Battle#conf term
Battle(config)#enable secret wheatgerm
Battle(config)#^Z

4,5,6, and 7. Configure an Extended IP access list on each router that first permits the desired traffic,
then denies the undesired traffic, then permits all other traffic. Make sure you end each access list
entry with the log keyword. Assign the access list as an incoming filter on each of the routers' serial
interfaces with the ip access-group in command. Here is an example of the procedure:

Battle>enable
Password:*******


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Battle#conf term
Battle(config)#no access-list 101
Battle(config)#access-list 101 permit tcp host 172.18.56.14 ...
any eq telnet log
Battle(config)#access-list 101 deny tcp any any eq telnet log
Battle(config)#access-list 101 deny icmp host 172.18.56.16 ...
any eq echo-request log
Battle(config)#access-list 101 permit ip any any
Battle(config)#int s0
Battle(config-int)#ip access-group 101 in
Battle(config-int)#int s1
Battle(config-int)# ip access-group 101 in
Battle(config-int)#^Z

The access list above does the following:

• Line 1 allows Telnet connections from the host IP address of 172.18.56.14.

• Line 2 drops all other Telnet traffic (Lines 1 and 2 meet lab objective #4).

• Line 3 drops ping requests from the host IP address of 172.18.56.16 (lab objective #5).

• Line 4 allows all other traffic to pass (meeting objective #7).

• All lines end with the log keyword (meeting objective #6).

Router Configurations
Corporate Router
!
!
hostname Corporate
!
enable password wheatgerm
!
no ip name-server
!
ip routing
!
access-list 101 permit tcp host 172.18.56.14 any eq telnet log
access-list 101 deny tcp any any eq telnet log
access-list 101 deny icmp host 172.18.56.16 any eq echo-request log
access-list 101 permit ip any any
!
interface Ethernet 0
no shutdown
description connected to Corporate LAN
ip address 172.18.56.1 255.255.0.0
keepalive 10
ip access-group 101 in
!


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interface Serial 0
no shutdown
description connected to Lincoln
ip address 172.19.1.2 255.255.255.252
encapsulation ppp
!
interface Serial 1
no shutdown
description connected to Battle
ip address 172.20.1.1 255.255.255.252
encapsulation ppp
!
router rip
network 172.18.0.0
network 172.19.0.0
network 172.20.0.0
no auto-summary
!
!
!
line console 0
exec-timeout 0 0
password oatmeal
login
!
line vty 0 3
password oatbran
login
!
line vty 4
password shellfish
login
!
end

Battle Creek Router
!
service timestamps debug uptime
service timestamps log uptime
!
hostname Battle
!
!
no ip name-server
!
ip subnet-zero
no ip domain-lookup
ip routing
!
!


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no shutdown
description connected to Battle Creek LAN
ip address 172.17.56.1 255.255.0.0
keepalive 10
!
interface Serial 0
no shutdown
description connected to Corporate
ip address 172.20.1.2 255.255.255.252
encapsulation ppp
!
interface Serial 1
no shutdown
description connected to Lincoln
ip address 172.21.1.2 255.255.255.252
encapsulation ppp
!
router rip
network 172.17.0.0
network 172.20.0.0
network 172.21.0.0
no auto-summary
!
!
!
line console 0
exec-timeout 0 0
password oatmeal
login
!
line vty 0 3
password oatbran
login
!
line vty 4
password shellfish
login
!
end

Lincoln Router
!
service timestamps debug uptime
service timestamps log uptime
service password-encryption
!
hostname Lincoln
!
!
no ip name-server
!


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ip subnet-zero
no ip domain-lookup
ip routing
!
!
no shutdown
description connected to Lincoln LAN
ip address 172.16.56.1 255.255.0.0
keepalive 10
!
interface Serial 0
no shutdown
description connected to Corporate
ip address 172.19.1.1 255.255.255.252
encapsulation ppp
!
interface Serial 1
no shutdown
description connected to Battle
ip address 172.21.1.1 255.255.255.252
encapsulation ppp
!
router rip
version 2
network 172.16.0.0
network 172.19.0.0
network 172.21.0.0
no auto-summary
!
!
!
line console 0
exec-timeout 0 0
password oatmeal
login
!
line vty 0 3
password oatbran
login
!
line vty 4
password shellfish
login
!
end

Tutorial


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Access Denied: Network Security with Cisco Routers for the CCNA
candidate
Introduction
Preparing for the CCNA exam is sometimes like eating at one of those "all you can eat" restaurants. There is an
abundance of study material to choose from, and it can be difficult to decide what you really need. If you spend
your whole time in the restaurant eating fried clams, you won't get the nutrition that you needed from that meal
and you'll probably end up with a stomachache. Likewise, if you spend all of your study time reading books
about TCP/IP, you will not get enough information to pass the exam and you'll probably wind up with a
headache. A good way to make sure that you get the nutrition that you need from eating is to create a meal plan,
with all of the foods selected to give you a nutritionally balanced meal. Then eat only the foods listed in the
plan, and only the amounts of each food that is specified in the plan.

The same approach can be taken when getting ready to take the CCNA exam. Consider the exam objectives
posted online by Cisco to be your CCNA meal plan. Study the material covered by the objectives listed. Some
material is stressed more heavily than other material on the exam, and that is reflected in the objectives. Some
concepts are weighted heavily on the exam, and consequently appear in the objective list frequently. Other
concepts are not covered much by the exam, and they appear only once or twice in the objective list. An
example of this is the Network Security section of the CCNA objectives list. Notice that there are only two
objectives under this heading. This would suggest that network security is not stressed too heavily on the exam,
and in fact, it is not.

But, do not think that you need not study this material! You do. The point is that you need to focus your study
time on the material covered by the objectives listed and not waste time with other tangential material. The
CCNA objective list is perhaps not as well organized as it might be. A close examination of the list shows that
some material that could be listed under the Network Security section is listed elsewhere instead. In this paper, I
will try to pull this material together and give you the information that you will need to know to answer the
questions you will see on the exam that relate to the following objectives:

• c1) Log into a router in both user and privileged mode

• c6) Control router passwords, identification, and banner

• f1) Configure standard and extended access lists to filter IP traffic

• f2) Monitor and verify selected access list operations on the router

Keep in mind that network security is an advanced discipline and really has a career path of its own. The
material presented here will help you to pass the CCNA exam, but there is certainly a smorgasbord of additional
network security material available. Now, let's get on to the meat and potatoes!

A very brief mention of security principles may help get us started. Much more discussion of this material is
available in some of the references at the end of this paper.


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Accessing real or virtual consoles, in formal security terms, is a problem of authentication: determining that the
purported user is actually who he or she claims to be. Most authentication systems are what security experts call
two-factor, the two factors being who you are and something only you know or have. "Who you are" is your
user ID, while the password is something you know. Other second factors include one-time passwords from
smart token cards or password lists, or biometric identifiers such as fingerprint or retinal scanners.

The general routing access lists described in this paper are means of access control: permitting or denying
traffic based on certain criteria. Access control lists, in general terms, consist of a pattern to match and an
action, such as permitting flow, that takes place when the pattern is matched.

A broader industry term is Access Control Lists, or ACLs. ACLs are available on both hosts and routers. A
complete security solution will use them in both places, as well as host-level authentication and other security
functions.

The Router Console
Most Cisco routers run the Cisco IOS software to perform all of their functions. The IOS interface with which
you interact is called Exec or the Command Line Interpreter (CLI). This is the command interpreter that accepts
your configuration commands and acts upon them. You can access the Exec command line in a number of
ways: through the console port, through a modem connected to the auxiliary port, or through a virtual terminal
session on one of the router's appropriately configured network interfaces.

IOS Exec Modes

Cisco IOS operates at different levels called Exec Modes. Each mode allows you to perform certain tasks. Once
you gain access to the Exec command line, you can perform some actions on the router, such as view the
version of the IOS software running on the router or look at the router's running configuration. To perform other
actions, such as change the configuration of the router, you must be operating in a different Exec mode. The two
Exec modes that you will be most concerned with are User Mode and Privileged Mode.

User Mode is the Exec mode that you are in when first accessing the Exec command line. It allows you to use a
limited subset of the IOS commands. To view the commands available to you in User Mode, simply type the
following at the command prompt:

Router>?

When you press enter, you will see a list of the IOS commands that are available to you in User Mode. One
command will be of particular interest to you. That command is enable. The enable command is used to enter
the next level of IOS privilege, called Privileged Mode.

In Privileged Mode, you have considerably more access to the router. You can access more system information
and operating statistics and you can change the global configuration of the router. From Privileged Mode, you
can configure the individual interfaces on the router as well as each of the protocols that the IOS software is
configured to support. To see a list of the IOS commands available in Privileged Mode, first use the enable
command to enter Privileged Mode and then enter the help command (?) as you did earlier in User Mode:


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Router>enable
Password:*******
Router#?

This time when you type the question mark and press enter, you will see many more commands listed than you
did when you performed this exercise in User Mode. From Privileged Mode you can enter other Exec modes,
like Global Configuration Mode or Interface Configuration Mode.

From a security standpoint, it is desirable to control access to the Exec command line itself, as well as to the
Privileged Exec Mode. This is done in a number of ways. Notice that when you used the enable command in
the example above, you were prompted for a password. The password protects access to the Privileged Mode on
the router.


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The password for Privileged Mode is set using either the enable Interpreting Prompts
password command or the enable secret command. The enable secret
command is used to create an encrypted password for access to Notice that the Privileged Mode Exec
Privileged Mode. The enable secret password is used by IOS versions prompt is a # sign. This is different
10.3 and above, and is preferred over the enable password password from the User Mode > sign. Pay
when both are configured. particular attention to the prompt to
determine in which IOS mode you
The enable password command is used to configure a password for are operating. Some of the questions
access to Privileged Mode. The password is not encrypted unless you do on the exam may be designed to catch
so manually. This command is used in IOS versions earlier than 10.3 as your attention to detail in this regard.
the only method to configure a Privileged Mode password. With version Watch out for answers that appear
10.3 and higher of the IOS, this command will configure a password correct because they have the correct
that will be used when no enable secret password has been configured. command syntax but are actually
You can configure the enable password password and the enable incorrect because the wrong Exec
secret password to be the same, but IOS will give you a warning when mode prompt is displayed. For
you do so. You can ignore this warning if you really want the two example, the following is an invalid
passwords to be the same. command line:
Both the enable password and the enable secret commands are Router>debug ip rip
Privileged Mode commands. The router must be in Global
Configuration Mode (accessed through Privileged Mode) for these You cannot execute the debug ip rip
commands to execute successfully. Of course, if no password has been command in User Mode. You must
previously configured, it should be no problem to get to the right Exec be in Privileged Mode for this
mode. The router's Initial Configuration Dialogue, if used, will prompt command to execute successfully.
you to set up a Privileged Mode password. Once a Privileged Mode The correct prompt would appear as
password has been configured, you will need to know what it is in order follows:
to change it to something else. Lost or forgotten passwords can be a bit
of a pain for router administrators. Password recovery, though beyond Router#debug ip rip
the scope of this paper, is well documented elsewhere for various IOS
versions and different Cisco hardware. It is well worth the effort to learn It is easy to overlook this type of
password recovery techniques for the Cisco devices that you must thing under the pressure of a live
support. exam. Remember to take your time,
stay calm, and read each answer
Securing Console Access carefully and you will be sure to spot
these kinds of detractors.
Once you have controlled access to the Privileged Mode with a
password, you will want to control access to the Exec command line itself. You will need to consider keeping
the router in a secure location to control access to the console port itself, and then use the line con 0 command
to configure the console port for a login password. Follow these steps to configure the console port to prompt
for a password before allowing access to the Exec command line:

Router>enable
Router#config term
Router(config)#line con 0
Router(config-line)#login
Router(config-line)#password cisco
Router(config-line)#^Z


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Securing Modem Access

Additionally, you will want to configure a login password for access to the Exec command line through remote
means. If you have a modem connected to your aux port, you will need to configure an auxiliary port password
to control access to the router through that port. To do this, you must first configure the aux port using the line
aux 0 command. This command is a Global Configuration Mode command that allows you to configure the first
auxiliary port (port 0). Follow these steps to get to the correct mode and configure the aux port with a password:

Router>enable
Password:*******
Router#config term ; puts you in Global Configuration Mode
Router(config)#line aux 0
Router(config-line)#login ; Now in Line Configuration Mode
Router(config-line)#^Z ; saves changes, exits Config Mode

Note: Do not actually set the password to "cisco" -- use a password that is more difficult to guess.

Securing Telnet Access

In addition to setting a password for the aux port, you will want to setup access to the Exec command line
through Telnet sessions to virtual terminal lines on the router. These virtual terminal (vty) lines allow you to
connect to the router through Telnet sessions to its network interfaces. The network interfaces must have the IP
protocol configured to support Telnet sessions. The router must also have its vtys configured, and you must
setup a vty password before the router will accept any incoming Telnet sessions. To do this, use the line vty 0 4
command as follows:

Router>enable
Password:*******
Router#config term
Router(config)#line vty 0 4
Router(config-line)#login
Router(config-line)#^Z

Cisco routers can accept up to 5 Telnet sessions (numbered 0 through 4) concurrently. The vtys for all of these
sessions are configured with line vty 0 4 command. The 0 and the 4 in the command indicate the first and last
session configured by the line vty command. You can configure each line individually by only specifying one
vty number in the command. For example, to configure only the fourth vty line, use the following command:

Router(config)#line vty 4

It is common practice to configure at least one vty with a different password from the others and to limit who
has access to this vty password so that there will always be an available vty when needed.

Router Identification

In the configuration examples used so far, you will notice that the prompt always begins with the word
"Router." This is the actual name of the router that we are configuring. If we want to change that name, we use

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the hostname command. It is a good idea to use a host name that is meaningful to anyone who needs to
administer the router, but that does not give away too much information to someone who accesses the router
without appropriate authority to do so. You might want the router's host name to indicate the location of the
router, or the router's role in the internetwork, but try to do so using some sort of naming convention rather than
stating it explicitly in the host name. For example, try the name fl3ar1 instead of the name
3rdFloorAccessRouter1. This may be considered a small point, but it is part of the overall security of the
internetwork. Each small component works with each other to form the composite security architecture -- in
other words, every little bit counts.

To configure the host name on a router, follow these steps:

Router>enable
Router(config)hostname fl3ar1
Router(config)^Z
fl3ar1>

Banner Messages

Another small component in the overall network security architecture is the router's banner. The banner is a
message that the router displays whenever you attempt to access the Exec command line. It might be tempting
to place a banner message that says something like "Welcome to the company's Cisco internetwork. Call the
help desk at 555-1212 for support." This is not a very good banner message, from a security perspective. There
have been cases where administrators have attempted to prosecute people who have accessed their
internetworks illegally, only to find that the case shot down by the perpetrator's claim that the company's banner
message gave them the impression that they were welcome there. A better banner message might be one that
indicates that unauthorized access will be prosecuted. Try something along the lines of the following:

"You have accessed a private internetwork. Unauthorized access to this internetwork is prohibited and will be
prosecuted in accordance with Title 18, U.S.C. -- if you are not explicitly authorized to access this internetwork,
log off now!"

To configure the router's banner, use the banner motd command. What the heck is "motd" you ask? It is short
for message of the day. In this particular case, unless you change it yourself daily, it is more like the message of
every day. To use this command successfully, you must specify a delimiter (of your choice) that indicates the
end of your message. It is common to use the # sign as a delimiter. Here is an example:

Router>enable
Router#config term
Router(config)#banner motd #
Enter TEXT message. End with the character '#'.
You have accessed a private internetwork.
Unauthorized access to this internetwork
is prohibited and will be prosecuted in
accordance with Title 18, U.S.C. -- if you
are not explicitly authorized to access
this internetwork, log off now!
#
Router(config)#^Z


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Configuring a banner message like the one above will provide an effective indicator regarding who is allowed
to access your internetwork to those who connect to your router either deliberately or accidentally.

At this point, we have covered all of the material in the first two objectives we listed in the introduction. This
material is fairly simple and straightforward. Now let's move on to material that is a bit more complex.

Access Lists
Password security is one level of an overall approach to securing your internetwork. Passwords are simple, but
are also a weak level of security. Passwords are often easy to guess, and even the most complex of passwords
can be derived given enough time. To take security to the next level, you will want to limit access to the router
on a per packet basis. To accomplish this on Cisco routers, you use access lists.

Access lists are not used for security alone. They also perform other useful functions on Cisco routers whenever
certain traffic must be identified on a per-packet basis. Access lists are useful when you need to filter traffic off
low-bandwidth links or to trigger certain events such as the initiation of a dial-up connection to another router.
This paper will focus primarily on the security uses of access lists, since access lists are specified in the
Network Security section of the CCNA exam objectives, although the basic principles for creating access lists
are the same regardless of what you are trying to accomplish with them.

An access list is a list of criteria used for identifying certain traffic, along with instructions for what action to
take when this particular traffic is found. When access lists are executed, traffic is compared to certain patterns
specified in the access list. When a match is found, then an action is taken as specified by the access list. The
action to take is either to permit the traffic (allow the packet to pass through the router) or to deny it (drop the
packet). Access lists are most often made up of a number of patterns for comparison, and packet information is
compared to each one in turn until either a match is found, or the end of the list is reached. Before looking at the
actual IOS syntax for creating access lists, let's examine how they work in general. Here is an example of an
access list:

Criteria to compare Action to take
Traffic destined for network X? Deny
Traffic destined for host Y? Permit
Traffic originating from host Z? Permit
Traffic using port N? Deny

In the preceding table is a list of criteria against which each packet will be compared. If the information in the
packet matches the criteria in the comparison, then the associated action will be taken. Using the table, if a
packet contains a destination address on network X, then it will be denied (the packet will be dropped). No
further comparisons will be made for this packet; it will simply be dropped because it matched the first line in
the list. If the packet is NOT destined for network X, then it will be compared to the criteria specified in the
next line. In this case, if the packet is not destined for network X, the router will determine if the destination
address is that of host Y. If it is, then the packet will be allowed to flow through the router. Again, no more


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comparisons will be made. Once a packet matches a line in the access list, the corresponding action is
taken, and no further comparisons are made.

It is very important to remember the point above because it makes the order in which you specify criteria in
your access list critical. Consider the following four hosts:

Hostname Address
Arthur network 1, host 1
Dipsy network 1, host 2
Kermit network 2, host 1
Kipper network 3, host 1

Assume that you have a router that connects network 1, network 2, and network 3, and that you are configuring
an access list on that router. The access list will filter traffic based on the following criteria:

Criteria to compare Action to take
Traffic destined for network 1? Deny
Traffic destined for host Dipsy? Permit
Traffic destined for host Kipper? Permit
Traffic destined for network 3? Deny

Read the access list criteria in the table above, and pay attention to the order in which they are specified. Can
traffic from network 2 or network 3 reach the host named Dipsy? The second line explicitly permits traffic
destined for host Dipsy, so you might think that traffic could reach Dipsy. The fact is, though, that no traffic
from networks 2 and 3 can reach the host Dipsy. Dipsy resides on network 1. Traffic destined for Dipsy will, of
course, also be destined for network 1 in order to reach Dipsy. Any traffic destined for network 1, including
traffic destined for Dipsy, will match line one of the access list. The action specified by line one is Deny, so the
traffic will be dropped. No further comparisons will be made for these packets! The traffic destined for
Dipsy will never be compared to the criteria specified in line two, because it will already have been dropped by
matching line one. When creating access lists, the order in which you enter your criteria is critical to the effects
of the access list.

What happens when a packet does not match any of the criteria specified in an access list? That is a very good
question. If a packet should make it past the last line of an access list and not match any of the comparison
criteria, the router needs to know whether to permit or deny it. The safest option, from a security perspective, is
to deny it, and that is what Cisco routers will do. There is a term for this default action at the end of an access
list -- "implicit deny any." It is never actually stated in the access list or printed in any configuration (hence the
"implicit" in the name), but it is a part of every access list on a Cisco router. The "implicit deny any" ensures
that any packet that does not match some explicitly stated access list criteria will be dropped once it passes the
end of the list.


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It is important to remember the "implicit deny any." If it is your intention to allow any traffic that does not meet
any of your stated criteria, you must add to the access list a line that explicitly permits this traffic. This line will
be the last line of the access list, and will permit the traffic before it is dropped by the "implicit deny any."

Types of Access Lists

Access lists are protocol specific. There are access lists for many different protocols, including TCP/IP,
IPX/SPX, AppleTalk, DECnet, and Banyan VINES. The CCNA exam will focus mainly on the access lists that
are used to filter TCP/IP traffic, although there may also be some questions on IPX/SPX access lists as well.

Standard and Extended IP access lists

IP access lists fall into two categories: standard IP access lists and extended IP access lists. Standard IP access
lists are able to examine the source address of an IP packet and take action based on the information found
there. Extended IP access lists offer much more flexibility. They can take action based upon a number of
different fields in the IP packet, including the source address, the destination address, and the port number.

Access lists are identified by number. The access list number also indicates the type. Standard IP access lists,
for example, are identified by a number within the range 1 to 99. Extended IP access lists are numbered from
100 to 199. This convention establishes a limit of 99 Standard IP access lists and 100 Extended IP access lists
on the router. This limit can be overcome using named access lists, which I will discuss later in this paper.

The access-list IOS command is used to create an access list. The syntax for a Standard IP access list is as
follows:

access-list number {deny|permit} source [source-wildcard]

Each line you configure for a Standard IP access list takes this form. The access-list command indicates that
you are creating an access list. The "number" parameter uniquely identifies the access-list on the router and
indicates its type. The {deny|permit} parameter indicates the action to take when a match is found, and the
"source" parameter indicates the criteria for comparison (in this case, a source IP address).

The [source-wildcard] indicates a mask to apply to the source IP address in order to specify which bits in the
address you care about matching. This mask, called the wildcard mask, allows you some granularity in
specifying a match based on the source IP address. You can specify that you want all of the bits to match,
indicating a specific host address, or you can specify that only certain bits need match, broadening the scope of
your filter to a range of host addresses or subnet addresses.


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Wildcard masks are often confused with IP subnet masks. They are Hint
similar in function, but opposite in approach. When an IP subnet mask is
"applied" to an IP address, all the bits in the address that correspond to For each octet, the sum of the
bits in the mask containing ones are considered part of the network equivalent subnet and wildcard masks
portion of the address, while the bits in the IP address that correspond to should be 255. For example, for the
the bits in that mask that contain zeros are considered part of the host subnet mask 255.240.0.0:
address. With a wildcard mask, any bit in the IP address that
corresponds to a zero bit in the mask must match the bit in the access list 255.240. 0 . 0 subnet mask
criteria exactly. Any bit in the IP address that corresponds to a one bit in + 0 . 15.255.255 wildcard mask
the mask will match the bit in the access list criteria, regardless of its ---------------
value. 255.255.255.255

Here is an example to make this clear:

access-list 1 permit 10.10.10.10 0.0.0.0

In this example, the source IP address is 10.10.10.10, and the wildcard mask is 0.0.0.0. This means that every
bit in the mask is a zero. In this case, every bit in the source IP address of the packet being examined must
match the bits in the address 10.10.10.10 in order for this line of the access list to be matched. This line
effectively filters traffic from a specific host: 10.10.10.10, and allows it to pass through the router (thanks to the
"permit" parameter). If I wanted to allow traffic originating from any host whose address contained 10 in the
first three octets, I would use the following line:



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Notice that we have changed the wildcard mask so that now the last Read the Fine Print
octet contains all ones. In this case, if a packet is examined, and each of
the first three octets of the source IP address contained the value 10, the One thing that you will notice as you
packet would be permitted, regardless of the value of the last octet. This look at access list lines is that it can
would obviously allow much more traffic to pass through the router than become difficult to distinguish the IP
the first example. You may also have noticed that we have changed the address criteria from the wildcard
"source" parameter to 10.10.10.0 -- the zero at the end is really only a mask. The numbers and the dots tend
placeholder at this point. Since the entire last octet of the wildcard mask to blend together. The only thing that
is set to ones, any value in the last octet will be declared a match. It keeps the source/source wildcard pair
makes no difference what we set this value to in the "source" parameter, 10.10.10.0 0.0.0.255 from being one
but it is conventional to set it to 0 in this case. We really don't care what long string is the missing period
the value is, and in fact the one bits of a wildcard mask are often called between the source IP address and the
the "don't care" bits. wildcard mask.
Suppose that I want to deny traffic from every host on the 10.0.0.0 Be careful when taking the exam that
network. The following line would apply: you read the lines of the access lists
carefully. Make sure that you are
access-list 1 deny 10.0.0.0 0.255.255.255
certain what the mask really is before
you select an answer to an access list
In this case we don't care what the host portion of the address is, so we
question. It can be easy to think that a
can set those bit positions in the wildcard mask to ones. We want to
zero at the end of the "source"
exactly match the value 10 in the first octet, so we set the first octet in
parameter is actually a zero in the
the wildcard mask to all zeros.
first octet of the wildcard mask.
Consider the following line:
This is one of those attention-to-detail
access-list 1 permit 0.0.0.0 255.255.255.255 issues again, but it can be a source of
frustration for test takers. Remember
Which traffic will this line permit? Examine the source/source wildcard to stay calm, take your time, and read
pair. The IP address of 0.0.0.0 is really meaningless in this case, because the question and answers carefully
the wildcard mask contains a one in every position. What we are before selecting an answer.
effectively specifying is that absolutely any address will match this line.
Cisco has created a shortcut that you can use when you want to specify this address/wildcard pair. The shortcut
is to use the any keyword. Using the shortcut we can rewrite the line like this:

access-list 1 permit any

This is a useful statement to put in an access list as the last line if you want to prevent traffic from being
dropped by the "implicit deny any."

Let's put a few lines together to make a more functional access list. Look at this example:

access-list 1 deny 10.10.10.10 0.0.0.0
access-list 1 deny 10.10.0.0 0.0.255.255
access-list 1 permit any


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Notice that each line above contains the same access list number (1). This indicates that each line belongs to the
same access-list, and that the access list is a Standard IP access list. When applying this access list, the router
will compare the IP source address in each packet to the criteria specified by the "source" parameter in each line
of the access list. Comparisons will be made to each line of the access list, one line at a time, in the exact order
in which they appear above, from top to bottom, until a match is made or until the "implicit deny any" is
reached. In the example above, the implicit deny any will never be reached, because all traffic that does not
match any earlier lines will be caught by the "permit any" in the last line of the access list.

What does this access list do? First, it denies any traffic from the host with the IP address 10.10.10.10. This is
accomplished with the 0.0.0.0 wildcard mask. Then it permits traffic from any host whose IP address contains
the value 10 in the first three octets. All of the first three octets must contain the value 10 for this line to be
considered a match, thanks to the 0.0.0.255 wildcard mask. Next, it denies any traffic from hosts whose IP
addresses contain the value 10 in the first two octets. The values in the third and fourth octets will not matter,
since we have a 0.0.255.255 wildcard mask. Finally, all other traffic will be permitted, thanks to the "permit
any" line.

Notice that as the access list is executed from top to bottom, the comparison criteria become more and more
general. This is the best way to build an effective access list. Since the order in which you enter your criteria is
critical, put your most specific criteria in the access list first, and then broaden your scope with each successive
line.

Once you have created an access list, there is no way to effectively edit it on the router, except to add additional
lines to the end of the list. If this is not what you intend to do, you should copy the router configuration to a file
and edit it with your favorite ASCII editor, then move it back to the router. Otherwise, you will need to
completely re-write the access list from scratch to edit it. Before you begin to create an access list with a given
number, or if you wish to completely re-write an existing access list, you should use the no access-list IOS
command. For example, if you wish to create access list number 37, or if you wish to re-write access list 37
from scratch, you should first use the command:

no access-list 37

This will make sure that any previously configured lines for access list 37 are removed from the router
configuration before you enter any new lines.

At this point in our discussion, it will be useful to briefly review some of the details of TCP/IP. This will not be
a thorough discussion of the protocol suite, only a review of some of the points that are relevant to the topic of
access lists. First, let's take a look at the IP packet format, which is illustrated in Figure 1.


350-018

Figure 1: IP packet format

When considering the IP protocol, there are a few things to keep in mind. First, IP is a connectionless protocol.
Second, in order for IP packets to be delivered correctly there must be a Source IP address and a Destination IP
address declared within the packet. Finally, there should be some way to indicate the higher layer protocol that
is to receive the IP packet's data. These objectives are accomplished by the Source IP address, Destination IP
address, and Protocol field of the IP packet header.

Extended IP Access Lists

So far, while using Standard IP access lists, we have only been concerned with the Source IP address field of
the IP packet header when making the decision to permit or deny. A quick examination of the packet format
above shows that there may be much more information of interest to us that would allow for much greater
flexibility in our filtering. In order for us to make use of this information, we need to use Extended IP access
lists. Extended IP access lists allow us to use the other fields in the packet header to make filtering decisions. In
the case of IP packets, we would be interested in the Destination IP address field as well as the contents of the
Protocol field.

It might be obvious why we find the Destination IP address field interesting. Just as we have been filtering
packets so far based on their source, we might also want to filter them based on their destination. Why do we
care what the contents of the Protocol field are? Extended IP access lists give us the ability to filter packets
based on information contained in the layer 4 header as well. The contents of the Protocol field in the IP header
will allow the router to determine what type of layer 4 header to expect.

Different higher layer protocols within the TCP/IP suite require different information to be specified within the
header at layer 4.The information that will be interesting to us when creating access lists starts with whether the
protocol is connection-oriented or connectionless. Connection-oriented protocols will use TCP as the layer 4
protocol.

Let's take a look at the TCP header:


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Figure 2: The TCP header

Some higher layer protocols within the TCP/IP suite use a connectionless layer 4 protocol called UDP. Here is
the UDP header:

Figure 3: The UDP header

In either case, the fields that concern us most are the Source Port number and the Destination Port number
fields. Each of the many higher-layer protocols within the TCP/IP suite has a port number associated with it.
This number is the mechanism that allows the transport layer to support multiple higher-layer protocols. The
port number uniquely identifies the upper layer process that is the source or recipient of any given piece of data.
Well-known port numbers are those numbers that are assigned by the Internet Assigned Numbers Authority
(IANA), and are documented in RFC 1700.

Some of the common port numbers that you may see include:

FTP TCP 21
TELNET TCP 23
SMTP TCP 25
DNS TCP 53, UDP 53
TFTP UDP 69
SNMP UDP 161

Figure 4: Some Well-Known Port numbers


350-018

In order to filter IP traffic based on the higher layer protocol in use, you would specify the TCP or UDP port
number associated with that protocol in your access list. Now that we have touched on the basics of ports, let's
move on and examine the specific syntax of Extended IP access lists.

The syntax for Extended IP access lists extends the syntax you've already learned for Standard IP access lists.
Initially, the syntax statement can be quite an eyeful, but after close examination it becomes easy to decipher.
There are slight variations depending on whether the protocol uses TCP or UDP, so we'll cover them each
separately. First we'll cover TCP. Take a look:

access-list access-list-number [dynamic dynamic-name [timeout minutes]] {deny | permit} tcp source source-
wildcard [operator port [port]] destination destination-wildcard [operator port [port]] [established]
[precedence precedence] [tos tos] [log]

Confusing? No problem. Let's look at all of the new keywords and parameters and define them. Once you
understand the purpose of each, this will no longer seem like such a mess. Keep in mind that not all of the
keywords are appropriate all of the time. The lines in your actual Extended IP access lists will not get this
complex very often. Here is a breakdown of the syntax statement above:

access-list: The IOS access-list command indicates that you are configuring a line in a particular access list.

access-list-number: The access list number uniquely identifies the access list on the router and also indicates
its type.

dynamic dynamic-name [timeout minutes]: The optional keyword dynamic indicates that this access list is
only valid for a limited time. The timeout parameter specifies the amount of time that an access list entry
remains in a dynamic access list. Dynamic access lists are outside the scope of the CCNA exam, and
consequently are outside the scope of this paper.


350-018

deny|permit: Specifies the action to take if the conditions in the access Lock and Key
list line are met.
Though dynamic access lists are
tcp: Specifies the protocol to examine. In this case, TCP is specified. beyond the scope of this paper, here
is a bit of information about them and
source: The IP address of the source station. a pointer to where you can find out
more.
source-wildcard: The wildcard mask to apply to the source IP address
for additional filtering. Dynamic access lists are a key feature
in what Cisco calls Lock-and-Key
operator: The operator field indicates which logical comparison you Security. The basic purpose of Lock-
wish to make with the value for the source or destination port. Valid and-Key Security is to provide access
operators are: to a specific source/destination host
through a user authentication process.
lt - less than
gt - greater than This process involves the creation of
eq - equal to appropriate access lists dynamically,
neq - not equal to as needed, and the removal of these
range - an inclusive range access lists after a predetermined
period of time.
The range operator requires that you specify two port numbers; all port
numbers between and including the values you specify will constitute a Basically, this is how it works:
match.
1. You initiate a Telnet session to the
The position of the operator and port number(s) indicates whether the router.
match is associated with the source or destination port. If they
immediately follow the source and source-wildcard keywords, then they 2. You are authenticated. (This is a
must match the source port. If they immediately follow the destination different authentication process than
and destination-wildcard, they must match the destination port. the simple vty password).

port [port]: The port parameter indicates the port number that will 3. The router creates an entry in the
constitute a match. In the case of a range, two port numbers must be dynamic access list.
specified -- a low and high value. The port parameter may be a number
between 0 and 65535. For many of the well-known port values, this 4. You do whatever it is that you
parameter may be a name instead of a number. Valid names include (but connected to the router to do and then
are not limited to) telnet, ftp, tftp, and domain. end your session.

5. The access list entry is removed.

Cool, huh? For more detailed
information regarding dynamic
access lists, refer to the Lock-and-
Key Security section of the
documentation for your version of
IOS. For version 11.2, it can be found
online.

( is not associated with Cisco.)

350-018

destination: The IP address of the destination station. Finding Ports
destination-wildcard: The wildcard mask to apply to the destination IP The Internet Assigned Numbers
address for additional filtering. Authority (http://www.iana.org/) is
the definitive source of port number
established: The established keyword causes a match to occur if the assignments. The most commonly
ACK or RST bits of the TCP segment are set. This would only occur if used port numbers are defined in
there has already been a session established. In the case of an initial http://www.isi.edu/in-
session request, these bits would not be set. notes/rfc1700.txt
precedence: Matches the value of the Precedence field of the IP header. In the formal port assignment
process, 0-1023 are assigned to "well
tos: Matches the value of the Type Of Service field of the IP header. known," standards-based services.
1024-2047 may be registered
log: Indicates that a message should be sent to the console when a match voluntarily by vendors who wish to
occurs. The log message will be sent to the console after the first match avoid conflict with other server port
of this line, and then a summary message will be sent at 5-minute numbers. Port numbers above 2047
intervals thereafter, indicating the number of packets that have matched usually -- but do not always --
this line in the previous 5 minutes. suggest client ports.
Now that you know what each keyword and parameter means, I am sure that you feel relieved. Extended IP
access lists are easy once you become familiar with them. Let's examine a few specific examples to help you put
all this information into practice.

Consider the following line:

access-list 101 permit tcp any any

This is perhaps the most basic form of an Extended IP access list. It is also probably useless in practice, as it
allows all tcp traffic to flow through the router, but it is useful here to illustrate that the syntax can be fairly
simple, depending on what you are trying to accomplish. Here is a more useful example:

access-list 101 permit tcp 200.199.198.0 0.0.0.255 any eq 23

The access list line above permits telnet traffic to any destination from any host on the 200.199.198.0 network.
How? Like this -- the access-list command indicates that we are configuring an access list entry. The number of
this particular access list is 101, indicating that we are configuring an Extended IP access list. The action to take
when a packet matches this line is to permit the traffic. The protocol we are to examine is TCP.

The source IP address and wildcard mask is 200.199.198.0 0.0.0.255. This indicates that in order to

Where to Filter
In addition to the syntax of the access list command lines, and the logical order of each access list entry, there is
another issue to contend with when using access lists. This issue is placement. Once you have identified the
traffic that you want to filter, you must consider the best location to place that filter.


350-018

Access lists are processor intensive for the router that executes them. You must make sure that the router you
plan to use for filtering has the processor capacity to handle the increased load that the access lists will place on
it.

Deciding where to place your access lists will be determined in large part by the purpose of the access list.
Consider this small scenario:

Let's say you simply want to prevent Host A from reaching Network C. You create a simple Standard Access
list to filter traffic from Host A. Where do you place this access list? You must place it on Router 2. If you place
the access list on Router 1, then Host A will not only be unable to reach Network C, but it will also be
prevented from reaching Network B. This is probably not what you intended. You might place the access list on
Router 2 as an outbound access list on the interface directly connected to Network C, and this would work. It
might be better, however, to place the access list on Router 2 as an incoming access list on the interface directly
connected to Network B. Why? This will deny the traffic before it enters Router 2. This reduces the impact of
the traffic on Router 2 by dropping it before it goes through the routing process. It makes little sense in this
scenario to allow the traffic to be routed only to drop it before it leaves the router.

With Standard IP access lists, placement is somewhat easier to determine due to the fact that they can only filter
traffic based on source address. Give your scenario a bit of thought, and it becomes easy to spot issues like
those we identified in the simple example above. Standard IP access lists usually must be placed as close as
possible to the destination in order to filter the traffic where it is unwanted, but allow that traffic to reach other
points within the internetwork.

Extended IP access lists offer greater flexibility in filtering, but this flexibility can make placement decisions
less obvious. Sometimes access list placement issues become a trade off between network bandwidth and ease
of administration. In general, placing Extended IP access lists close to the source of the traffic is a good idea in
order to keep traffic that will ultimately be dropped from travelling too far through the internetwork and using
up valuable bandwidth. This also helps to reduce the impact of ICMP messages that might get sent to the source
in response to the traffic getting dropped. These messages include Host Unreachable and Network Unreachable
messages.

On the other hand, it can simplify administration to place access lists closer to the core of your network rather
than distribute them out to the edge of the network. Locating the access lists closer to the core might enable you
to consolidate them, giving you fewer lists to maintain. This becomes an even more important consideration as
your internetwork grows.

Access Lists' Impact on Performance


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As mentioned earlier in this paper, access lists can be very CPU intensive for the router that executes them. In
addition, access lists have an impact on a router's switching function. Autonomous switching cannot be
performed on any interface that uses Extended IP access lists. Likewise, silicon switching cannot be performed
on interfaces configured to use dynamic access lists. These issues need to be considered when you decide
whether to use access lists and where to place them.

Monitoring Access Lists
It is often useful to examine the contents of the access lists on your router. To do this, use the IOS command
show access-list. This command, by itself, will display the contents of all access lists that are configured on the
router, regardless of type, name, or number. To view only IP access lists configured on the router, use the show
ip access-list command.

To view a specific IP access list, use the access list number with the show ip access-list command. For
example, to view only the contents of access list 101, use the following command:

Router#show ip access-list 101

Conclusion
If you have read this entire paper and absorbed the material presented within it, you know now all that you will
need to know to correctly answer the questions on Network Security that you will find on the CCNA exam. You
may even feel comfortable setting up access lists on your own routers. Your comfort will increase as you use
them and become more familiar with their application, their impact, and their performance.

You will be asked to solve Network Security problems on the CCNA written exam. Are you ready to solve
them?

References
Cheswick & Bellovin (1994). Firewalls and Internet Security: Foiling the Wily Hacker. Reading, MA:
Addison-Wesley-Longman.

Chapman & Zwicky (1996). Building Internet Firewalls. Sebastopol, CA: O'Reilly.

RFC 2196. Site Security Handbook.

Stevens (1994). TCP/IP Illustrated, Volume 1 The Protocols. Reading, MA: Addison-Wesley-Longman.

Chappell (1999). Advanced Cisco Router Configuration. Indianapolis, Indiana: Cisco Press, Macmillan
Technical Publishing.


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